Olivero wrote:Well. You could also do some stitch welds on the plate to the table, then when done, cut it off and grind the welds down. I have heard of that working really well.

This only contains distortion until you cut it loose. The shrinkage occurred anyway. Every heard a tack go "SNAP" half-way through a weld? Why did it do that? Because welding metal affects it in some substantial ways.

This is a bit out of my league but I read pre-stressing as in shimming the plate edges and clamping for a slight concave prior to welding to compensate for heat deflection as one suggestion. Curious what would happen if you drilled a hole in the plate centered under the pipe min. 1/2in . Place the plate over a perimeter frame and drop the bolt thru the hole. Under the frame would need a solid cross piece strong enough to tighten the bolt to concave the plate as required. Weld on pipe and after cool release bolt. Have no idea if this would work. If it doesn't work beat the piss out of it with a hammer!!